The
complex relationships between atmospheric carbon dioxide, biological
productivity and the role of the Southern Ocean in carbon sequestration
have been demonstrated by scientists at NOAA’s Geophysical Fluid
Dynamics Laboratory and Princeton University in Princeton, N.J.

Ocean
waters that move toward the Antarctic continent sink as their temperatures
drop. Once this occurs, these waters then move northwards. New research
demonstrates that water at greater depths have a significantly different
impact than the high-nutrient waters that flow northwards at intermediate
depths. The circulation in the regions around Antarctica where water
sinks to depths greater than 1.5 km was shown to be largely responsible
for controlling the air-sea balance of carbon dioxide (CO2). The circulation
in the Subantarctic regions that feed water to depths between 0.5
and 1.5 km controls biological productivity. This research builds
on recent studies showing that different parts of the Southern Ocean
have responded differently to climate change.

Scientists
looked at atmospheric CO2 and tiny marine plants known as phytoplankton,
which remove almost 60 billion tons of carbon from the surface ocean
each year. Much of the carbon is recycled by other organisms within
the surface layer of the ocean, but about 10 billion tons of this
carbon sinks into the deep ocean. This “biological pump”
in the Southern Ocean is known to play a central role in the how much
CO2 is contained in the atmosphere, as well as the global nutrient
cycle.

“Even
though it’s a long way from occurring, we’ve long known
that the Southern Ocean could change Northern Hemisphere climate by
changing atmospheric carbon dioxide” said Anand Gnanadesikan,
research oceanographer at GFDL and a co-author of the paper, which
is published in the recent issue of the journal Nature. “What
we didn’t understand was the importance of the details of circulation.”

Gnanadesikan
worked with lead author Irina Marinov (currently a fellow in the NOAA
Postdoctoral Program in Climate and Global Change), and with GFDL
oceanographer J.R. Toggweiler and Jorge Sarmiento of Princeton University.
The team used a series of ocean model simulations to investigate which
areas of the Southern Ocean control atmospheric carbon dioxide levels,
which areas control biological production, and whether there is a
link between the two.

Using
GFDL’s Modular Ocean Model coupled with a biogeochemistry model,
researchers increased biological productivity in different parts of
the Southern Ocean, depleting surface nutrients and forcing more CO2
into the ocean. They were able to discern which location determined
the impact on air-sea carbon exchange. Depleting nutrients in the
Antarctic zone was up to 12 times more effective at reducing atmospheric
carbon dioxide as depleting nutrients in the Subantarctic zone.

“The
work we had done previously considered the Southern Ocean as a single,
relatively uniform ecosystem,” Gnanadesikan said, “but
J.R. Toggweiler had the idea that this was too simplistic - that more
attention needed to be paid to the Antarctic. We decided to put his
idea to the test.”

The
research confirmed that the key region for understanding atmospheric
carbon dioxide is not the relatively large area to the north of the
polar front, but the much smaller, poorly sampled region around the
Antarctic margin. This biological divide separating the Antarctic
from the Subantarctic suggests that one area could be modified –
by climate change or human intervention – without greatly altering
the other. These results have important implications for understanding
both past and future variations in atmospheric carbon dioxide and
biological production.

The
Geophysical Fluid Dynamics Laboratory advances NOAA’s expert
assessments of changes in national and global climate through research,
improved models, and products. The goal of GFDL's research is to understand
and predict the earth's climate and weather, including the impact
of human activities.

The
National Oceanic and Atmospheric Administration, an agency of the
U.S. Commerce Department, is dedicated to enhancing economic security
and national safety through the prediction and research of weather
and climate-related events and providing environmental stewardship
of our nation’s coastal and marine resources. Through the emerging
Global Earth Observation System of Systems (GEOSS),
NOAA is working with its federal partners and more than 60 countries
to develop a global monitoring network that is as integrated as the
planet it observes.